For the last 11 years or more, the conventional dried reagent approach to clinical chemistry (which does not use liquid or reconstituted liquid reagents in a cuvette) has been the dried slide test element technology such as that used by Eastman Kodak Company under the trademark "Ektachem" slides. In such technology, a dried reagent is supplied along with a binder in a reagent layer on a slide-like element. To assist in spreading the liquid uniformly into an area that then flows into the reagent layer, a spreading layer is usually provided above the reagent layer, for example, that taught in U.S. Pat. No. 3,992,158. However, the reading of the elements is via reflectance from the opposite side, and every effort is made to keep the detection light from penetrating the spreading layer, since the detectable color change occurs in the reagent layer(s).
Although such test elements work admirably, as evidenced by the billions of slide elements sold since Eastman Kodak introduced the product, it would be advantageous to provide a dried test element that does not require the various reagents required in, e.g., the aforesaid '158 patent and the related test elements.
Some efforts have been made to develop spectroscopic analysis of liquids into a quantitative science. Attempts were made to use impervious supports on which a liquid sample is spread. However, for quantitative analysis of most biological liquids, this is unsatisfactory since the constancy of the light-path as required by Beer's law cannot be easily maintained on an impervious support. A similar objection exists for early pervious supports that were tried, e.g., paper and thin layer chromatography elements. That is, although the depth of the liquid scanned is more maintainable in such pervious supports, they lack uniform three-dimensional porosity since the liquid does not spread uniformly in all directions throughout a known volume.
The most recent attempt at spectroscopic analysis is the use of microporous polymers to create a porous sheet for IR spectroscopy, for example, as taught in WO 93/00580. Although this purports to provide for wavelengths of detection that cover NIR as well as IR (p. 1, lines 25-28), where NIR as used therein is from about 750 nm to about 3000 nm, in fact the rest of the teaching is for IR only (above 3000 nm). The reason is, that it is known in the literature that the sheet materials listed in WO 93/00580 are unable to function quantitatively in the NIR, particularly in the reflective mode where the sheet must be at least 95% reflective. All of those materials listed (polyethylene, polypropylene, poly(tetrafluoroethylene) (PTFE), ethylene/propylene copolymers, poly(vinyl fluoride), polyester, chlorotrifluorethylene polymer, and Nylon), with the exception of PTFE, are not at least 95% reflective. PTFE does not however have the porosity required--it is not "sufficiently porous" as set forth in the Summary hereinafter.